This invention relates to a method of precipitating zirconium sulfate and, more particularly, a method for precipitating zirconium sulfate from aqueous solutions contaminated with large amounts of aluminum. This invention is particularly useful in the production of nuclear grade zirconium.
Zirconium and zirconium oxide are commercially produced from zircon ore via the well known Kroll carbochlorination process. This ore is generally contaminated with relatively large amounts of hafnium and iron and lesser amounts of other metals, most importantly up to about 3000 parts aluminum per million parts zirconium, by weight (or 3000 ppm Al/Zr). In addition, the zirconium produced by commercial grade production processes will also be contaminated with up to about 3000 ppm Al/Zr because aluminum tends to track with the zirconium.
The production of nuclear grade zirconium requires that the aluminum contamination of zirconium be reduced to less than 60 ppm Al/Zr. In about 1954 the United States Bureau of Mines ("USBM") developed a process for producing nuclear grade zirconium which utilizes a sulfate precipitation step to purify the zirconium. This process is described in USBM Report R15214. In USBM type of zirconium sulfate precipitation processes, a hot raffinate containing from about 0.5M up to about 2M zirconium tetrachloride or oxychloride at about 90.degree. C. from a solvent extraction step (for separating hafnium and iron) is mixed with sulfuric acid or ammonium sulfate. The pH of the mixture is then adjusted to about 1.5 via the addition of an ammonia-containing solution. Acidity control is important in these processes because high pHs result in unacceptably high co-precipitation of aluminum and zirconium and low pHs result in unacceptably high zirconium losses. The S/Zr ratio of the solution is generally maintained at about 0.4.
The undesired precipitation of aluminum which occurs at transient high pH regions upon the addition of ammonia has been traditionally controlled by providing high levels of dilution for reducing the pH and long digestion times for dissolving the aluminum. Thus, dilution water is frequently added with the ammonium solution to prevent the undesired co-precipitation of zirconium hydroxide and aluminum hydroxide in transient high pH regions. Steam injection is frequently employed to maintain a reaction temperature of about 90.degree. C. to about 100.degree. C. and to dilute the ammonium concentration.
The precipitate is then filtered in a drum or other filter and washed to produce a zirconium sulfate cake contaminated with less than 60 ppm aluminum. Depending upon the amount of wash water, the aluminum levels of the cake may be reduced to less than about 10 ppm aluminum.
Although the prior art precipitation processes produce nuclear grade zirconium on a commercial scale, these zirconium sulfate precipitation processes are very sensitive to process variables and occasionally produce precipitates which cannot be effectively filtered. If the pH is too high or if the digestion time is too long, zirconium hydroxide may dehydrate and form gelatinous zirconium oxide-containing precipitates. Also, the precipitate may have an high aluminum content. The filtration step will in such cases require the use of hydrochloric acid or other suitable acid or additional wash water to dissolve the aluminum or wash the precipitate or otherwise cure the process condition.